Differential pressure responsive apparatus

ABSTRACT

A flowmeter for converting differential high and low fluid pressure signals into an electrical signal includes a flexible compliance bellows and an overload valve actuating bellows. The high and low differential fluid pressure signals are applied respectively against the inner and outer surfaces of each of said bellows. A spring extends between the compliance bellows and a wafer made of a single crystal silicon material. The wafer includes a diffused fully active resistive bridge pattern thereon and is so arranged that distortion thereof caused by displacement of said compliance bellows results in unbalancing of said bridge and the production of the electrical signal. Adjustable stops are employed whereby the displacement of the compliance bellows is restricted to predetermined differential fluid pressure operating ranges thereby limiting the forces transmitted by the compliance bellows and spring to the wafer to a level below those tending to rupture said wafer.

United States Patent [191 Whitehead, Jr. et al.

[ Dec. 25, 1973 DIFFERENTIAL PRESSURE RESPONSIVE Primary ExaminerDonaldO. Woodiel APPARATUS Attorney-Arthur H. Swanson et al.

[75] Inventors: fioiiiertJC. \ghtil'tlehteagi, .l]r.;dLi)s'ter R. l 57 1ABSTRACT o 0 re an A flowmeter for converting differential high and lowAssignee! Honeywell Inc, Minneapolis Minnfluid pressure signals into anelectrical signal includes [22] Filed: Man 31, 1972 a flexiblecompliance bellows and an overload valve actuating bellows. The high andlow differential fluid [21] P N05 240,100 pressure signals are appliedrespectively against the inner and outer surfaces of each of saidbellows. A 52 US. Cl 73/398 AR, 73/407 R Spring extends between thecompliance bellows and 51 Int. Cl. G01l9/06 a Wafer made of a SingleCrystal Silicon material The [58] Field of Search 73/398 AR, 398 c,wafer includes a diffused fully active resistive bridge 73/407 R, 3353]) pattern thereon and is so arranged that distortion thereof caused bydisplacement of said compliance [56] References Cited bellows results inunbalancing of said bridge and the UNITED STATES E S production of theelectrical signal. Adjustable stops 3,559,488 2/1971 Weaver 73/398 ARare employed whereby the displacement of the 3343'420 9/1967 Kondo ct73/398 AR pliance bellows is restricted to predetermined differ3,400,588 9/1968 O'Connor 73/407 ential fluid Pressure Operating rangesthereby limiting 3.641,8l2 2/1972 Vick 73/88.5 SD the forces transmittedby the compliance bellows and 2,977,991 4/l96l Bauer 73/407 pring to thewafer to a level below those tending to rupture said wafer.

10 Claims, 4 Drawing Figures 7 I380 use 122 6202885 B; 8H0 I6 72 76 1%52I38 44 22 l2 l4 |66l68 l4l I42 I72 I76 I74 l7 I62 '0 \ll l il l l ll H lW9 I I :l8l

s r v v l- DIFFERENTIAL PRESSURE RESPONSIVE APPARATUS BACKGROUND OF THEINVENTION In order to measure the flow rate of a fluid it is necessaryto take a differential fluid pressure measurement of the drop inpressure which occurs in this fluid as it flows through an orifice in aflow line. To do this it has heretofore been necessary to resort tobulky, differential pressure measuring devices, e.g., inverted bellswhose bases are sealed off by a liquid in a tank and which are joinedtogether at their outer closed ends by a beam that in turn is mountedfor see-saw motion on a knife edge pivot bearing.

Many other similar bulky devices, e.g., ring balanced type flow metersthat contain a liquid have been employed. The liquid in these meters isdisplaced by a differential pressure of a fluid under measurement thatis on opposite sides of an orifice in a flow. line and the ring rotatesabout a knife edge pivot bearing to indicate differential pressureswhich extend between e.g. the zero and fifty inches of water range orthe zero and five inches of water pressure range.

PROBLEMS One problem encountered in present day meters of theaforementioned type is that users of such meters are now requiring themto be provided with expensive jeweled parts to reduce friction and to bemade more reliable, more compact, accurate and lighter, and more stableto reduce friction.

The aforementioned requirements are necessary because it takes only avery small amount of friction from any one or more parts of these belljar or ring balanced type meters to produce an inaccurate pressureindication.

Since the construction of bell jar and balancing ring type meters do notallow redesign of their parts in order to reduce the size of thesemeters another type of meter construction is therefore required in orderto achieve the previously mentioned more reliable, more compact,lighter, stable and accurate present day specification requirements forthese meters.

SUMMARY OF THE INVENTION One solution that has been found to eliminatethe aforementioned problems is to employ a semiconductive wafer having aresistive bridge diffused in a doped piezoresistive pattern thereon as asensor to sense changes occurring in the difference between separatehigh and low fluid pressures that are each applied to a differentopposite side of a compliance bellows and which in turn is connected byway of a resilient member to the wafer. 7

Maximum use of the space within the meter is accomplished by mounting abellows operated overload valve and the aforementioned compliancebellows in parallel relationship with one another so that a jointmovement can take place in these bellows as will hereinafter bedescribed in detail.

The use of a compliance bellows in the aforementioned uniquelyconstructed meter provides a correct output signal indication when anoverload pressure condition is present in either the low or high fluidpressure side of the meter. Without the presence of a compliance bellowsdifferential pressure measuring meters have heretofore been known toprovide false output indications which would lead the operator tobelieve that the differential pressure he was measuring was in a normaloperating range, where, in fact, a dangerous condition existed in whichthe fluid pressure on either the high or low side of the compliancebellows had reached an overload condition as is disclosed in thecopending J 9129 A- e vers: 1 -v Patfi t N -til i tlt n Anotherdesirable important feature which the unique transmitter disclosedherein employes is that the force exerted on the semi-conductor wafer islimited by adjustable stops. These stops are employed to limit themotion of the compliance bellows within a desired safe prescribed rangeof motion so that the force that the bellows actuating spring can applyto the semiconductive wafer will never exceed a preselected level whichcould cause the wafer to rupture.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the presentinvention may be had when the following detailed description is read inconnection with the accompanying drawings in which:

FIG. 1 shows a sectional view taken through one form of the differentialpressure sensing meter in which the wafer is of a beam shapedconfiguration and a U- shaped spring is employed as a displacement toforce converting means between the compliance bellows and the wafer andalso shows how adjustable, wafer protecting stops are employed toselectively restrict the range of motion through which the bellows canbe displaced;

FIG. 2 shows a plan view of FIG. 1 and how electrical jump wires areemployed to connect the diffused fully active resistive bridge on thesilicon wafer with an electrical head frame connection;

FIG. 3 shows a sectional view taken through another form of adifferential pressure sensing meter in which the wafer is an upper wallof a chamber and a coil shaped spring is employed as a displacement toforce converting means between the compliance bellows and the wafer andalso shows how adjustable wafer protecting stops can be employed toselectively restrict the range of the motion through which the bellowscan be displaced and FIG. 4 shows the electrical bridge employed toproduce an electrical signal that is responsive to the magnitude of thedifferential fluid pressure under measurement.

A DETAILED DESCRIPTION FIG. 1 shows a pressure sensing apparatus 10having a pressure-sensor 12 that is a wafer and which is made of singlecrystal silicon materialinto which has been diffused a fully activeresistive wheatstone bridge pattern 14.

A high pressure fluid l6 and low pressure fill fluid 18 both of whichare in the form of an incompressible nonconductive liquid are shownretained within the body 20 of the meter 10 and between high and lowbarrier diaphragms 22, 24 that are matched in size and stiffness tominimize the effect of temperature and static pressure.

The aforementioned fill fluids 16, 18 are shown positioned to applyfluid pressures that are in opposition to one another to opposite sidesof a flexible overload bellows 26 which forms a portion of the highpressure overload valve 28.

The aforementioned fill fluids 16, 18 are also shown applying pressuresthat are in opposition to one another to opposing sides of the overloadcompliance bellows 30.

Ports 32, 34 shown on the left side of the body 20 are provided throughwhich the aforementioned fill fluids 16, 18 are applied to fill theinterior of the meter while air is evacuated from the meter 10.

After fifliasthwastsr .lfll lliklli9fl l f .1 ball seal and plug units36, 38; 40, 42 are then inserted into the outer ends of the ports 32, 34of the body to completely seal off the till fluids retained within theinterior of the meter 10 from atmospheric pressure.

High and low process fluid pressures 44, 46 whose differential pressureis to be measured are applied by way of their respective passageways 48,50 and chambers 52, 54, formed by the high pressure heads 56, 58 andtheir separately associated diaphragms 22; 24.

The outer open end of the flexible bellows 26 of the high pressureoverload valve 28 is shown'fixedly connected, for example, by a solderto the inner wall of the ring shaped spacer 60. The lower flat surfaceof this ring 60 is retained in fixed relation witn an O-ring seal 62 anda surface forming the base 64 of a cylindrically bored out aperaturewall 66 formed in the body 20 of the meter 10 by means of a sleeveshaped spacer 68 and associated retainers 70, 72.

These retainers 70, 72 are shown fixedly connected to the top portion ofthe body 20 by means of threaded screw connections 74, 76.

The closed end of the overload bellows 26 has a cylindrical T-shapedblock 78 extending downwardly therefrom and has a resilient O-ring 80retained in teasin rsls sashisthstswith. ati slqvs The lower portion ofthis overload valve 28 has a rod 82 that extends in an upward directionin spaced apart relationship thro u gh 5" passageway 8 4 formed in thebody 20 of the meter 10. The upper end of the rod 82 is shown threadedlyconnected at 85 to the center of the lower portion of the cylindricalblock 78 for joint movement therewith.

A cylindrical block 86 forms an integral central portion of the rod 82which has a resilient O-ring 88.

The lower end of the rod 82 has a threaded end portion 90 on which thereis threadedly mounted a hub portion 92. An S-shaped spring 94 is mountedon the hub portion 92 and held in fixed relation thereon by means of acup shaped spring washer 96. This washer 96 is of the type having aradial slot for rapid assembly into the position as shown in FIG. I. Theouter ends of this S-shaped spring 94 are retained by means of two screwthreaded connections 98, 100 and associated spacers 102, 104; 106, 108.

When the head part of hexagonal hub portion 92 is rotated in onedirection, it will'cause the O-ring 80 and the parts associatedtherewith to be moved to an adjustably fixed operating position in adownward direction from the position shown and when it is rotated in theopposite direction it will cause the O-ring 80 to be moved to adifferent adjustably fixed operating position in an upward directionfrom the position shown.

The aforementioned adjustment is for the purpose of allowing an operatorto select the pressure level at which either the overload high or lowpressure fluid under measurement 44, 46 and its associated fill fluid 16or 18 can close the overload valve 28.

During the normal non-overload condition the O- rin gs 80 and 88 willremain in spaced apart relation with respect to their associated seats110, 112.

During an abnormal high pressure overload condition the pressure of thehigh pressure fluid 44 under measurement is raised to an abnormally highpressure level that is beyond the normal high pressure operating rangeof the meter 10.

This abnormal increase in the pressure of the high pressure fluid 44 isapplied to the high pressure barrier diaphragm 22, to the fill liquid 16and to the overload bellows 26. This action will cause the bellows 26and the block 78 attached to the upper end of the bellows 26 and theO-ring 80 connected thereto to move in a downward direction until theO-ring 80 is brought into an initial substantially non-deformable sealtight contact with the seating surface as is best shown in FIG. 1.

This initial sealing action traps a portion of the low pressure fillfluid 18 inside bellows 26, passageway 34, 114, the passageway 116between the bored out cylindrical wall 118 and the inner wall of thecompliance bellows 30, and within the space formed by the internalsurface of the bellows 30. The pressure of the trapped volume of therelatively incompressible fluid 18 is therefore initially raised toeffect a lowering of the differential pressure acting across thecompliance bellows 30 as the initial abnormal change in the highpressure fluid 44 and the high pressure fill fluid 16 occur. Thispressure vhegating action thus prevents the increase of the highpressure fill fluid 16 from rupturing the compliance bellows 30.

ljuring the movement of the overload valve 28 from the substantiallynon-compressed sealed position of the O-ring 80 to a compressed sealedposition with seating surface 110 a reduction in the volume of thetrapped low pressure fluid till 18 that is within the overload bellows26 will take place. Under this latter mentioned condition the magnitudeof the pressure of the high pressure fluid 44 and the high pressurefluid 16 is sufficiently high to cause the overload bellows 26 to becompressed and its associated block on which the O- ring 80 is mountedto move in a downward direction and the O-ring to be deformed andbrought into fluid seal tight engagement with sealing surface 110. Thebase portion 120 of the metal block 78 under the latter mentionedcondition is seated against the metal seating surface 110 of the body20. While the aforementioned compression of the bellows 26 and O-ringseal 80 takes place, a decrease in the volume of the low pressure liquidfill 18 within the overload bellows 26 will also occur as the overloadbellows, including its solid cylindrical end portion 122, is moved in adownward direction in a piston-like manner. This piston-like effect willthus occur during the downward movement of the bellows and while itsconvolutions are being brought into more compact relationship with oneanother.

More specifically, it should be noted that while the O-ring 80 is beingsqueezed and it is simultaneously moved between its non-compressed toits compressed sealed with the sealing surface 110, the top of thebellows 26 will move the same distance as the amount this O-ring 80 issqueezed, since these parts are rigidly connected. It should further benoted, since the area of the bellows 26 isgreater than that of theO-ring 80, a change in the volume of the trapped fluid results. Thischange in volume would normally cause an excessive change in pressure,but this is prevented by the expansion of the compliance bellows 30.

FIG. 1 also shows a plate 124 forming the top of the compliance bellows30 as having a boss 125 extending upwardly away from and downwardly intoa central portion of the bellows 30.

The bottom portion of the boss 125 has a stop plate 126 fixed thereto bymeans of a threaded screw member 127. The stationary base portion 128 ofthe compliance bellows 30 has a tubular portion 129 extending upwardlytherefrom for supporting an adjustable stop member 130 in a selectedposition thereon by means of the threaded connection 131 formed betweenthese parts. The internal wall of the tubular portion 130 forms apassageway through which the fluid 18 passes into the interior of thebellows 30.

The base 128 of the bellows 30 is fixedly connected by solder to thelower end of the hollow cylindrical support block 132. A passageway 128ais formed therein to allow free flow of the fluid 18 into and out of thebellows 30.

The support block 132 supports a sandwich type seal 133, comprised of anO-ring positioned between two rings made of a material trademarkedTeflon, in its upper external wall surface. This seal 133 extendsbetween the block 132 and the cylindrical wall 118 and forms a sealbetween the high pressure fluid 16 located in the upper portion of themeter and the low pressure fluid 18 located at the bottom portion ofmeter 10.

As is best shown in FIGS. 1 and 2 the block 132 is supported in fixedrelationship on the body of the meter 10.

This is accomplished as shown in FIG. 2 by means of a hold down strap134, positioned in the top recessed wall portions of the body 20 and apair of threaded screw connections 135, 136 passing through the strap134 into the top wall portion of the block 132. A pair of insulatorse.g. 137, 138 mounted on each of the screw connections 138a, 13812 alongwith the back up rings of seal 133 are employed, as shown in' FIG. 1, toelectrically isolate the bellows 30, a U-shaped spring nn om xt nd qaP521899! -tlx 2$$. Z I. the U-shaped member 140 connected by brazing tothe spring 139, the wafer 12 and the semi-conductor wafer 12 from themeter body 10. This construction prevents leakage current from passingfrom the wafer to ground.

A suitable metal shield 141 is retained as is best shown in FIG. 1,138a, 138b, to the support block 132 by screw connections. This shield141 as shown in FIG. 1 is connected electrically to the substrate ofsemi-conductor wafer beam 12 to prevent drift of the beam outward due toa condensor effect taking place between the fluid l6 and the barrierdiaphragm 22.

The beam shaped semi-conductor wafer 12 is in turn connected by a goldbonding material to a plate 142 which plate in turn is connected to thetop of the support block 132 by the screw connection 143, 144.

From the aforementioned description of the elements it can be seen thatas the high and low pressure of the respective process and fill fluids44, 16; 46, 18 apply varying pressures to the opposite sides of thecompliance bellows 30 the bellows 30 will be displaced in a vertical upand down direction. The movement resulting from this displacement willbe transmitted by way of the U-shaped spring 139 into a resulting forcethat is applied by way of the connection 140 to the semiconductorsilicon beam 12. The gradients selected for the spring 139 and bellows30 and the proper selected threadedly adjusted position for the stop 130are such that the meter can be set so that it can transmit only desiredforces to the semi-conductor beam 12. These forces will lie within adesired safe rnage of force levels that will not exceed any ultimateforce level that would rupture the wafer 12 and cause the electricalsignal transmitting characteristics of the resistive bridge portionthereon, 14, to be impaired.

It should be noted that the two heads 56, 58, the centrally located body20 and the support plates 146, 148 to which the barrier diaphragms 22,24, are fixedly attached are of a substantially square shapedconfiguration.

Furthermore, the heads 56, 58, body portion 20 and support plates 146,148 are connected in fluid seal tight relationship with one another as asingle unit by means of suitable O-ring seals 150, 152, 154, 156 andwith the aid of a suitable number of bolt and nut connections, forexample, 158, 160; 162, 164.

It should also be noted that a suitable number of fine wires, forexample, 166, 168, are connected as jump wires to an electrical headframe 172 between the active resistive bridge 14 formed on the siliconsemiconductor wafer 12 and upper multi-pin connections 174, 175, 176,177, 178, 179, 180, 181 ofa cylindrical feed through header 183. Thisheader 183 in turn is shown in sealed fluid tight engagement with thecylindrical bored out wall 184 formed in the body 20.

Additional leads 186 which may be mounted in the form of anotherflexible cable are connected to the lower multi-pin connections 188 ofthe feed through member 183 and passing downwardly through thecylindrical bored out portion 190 in the body 12 and extending outwardlythrough a cylindrically bored out port 192 formed on the right side ofthe body 12.

FIG. 4 shows that the fully active resistive wheatstone bridge pattern14, previously referred to under the description of FIG. 1, receivespower from a power source 194 that is external to the meter 10 shown inFIG. 1.

FIG. 4 shows that an output of the temperature compensated amplifiedunit 196 and the constant current regulator 198 is provided for thebridge 14 that is external to the meter 10.

A more thorough detailed description of the amplifler unit 196 and howit is interrelated with the output of the bridge 14 to providetemperature compensation can be obtained by referring to the Anthony M.Demark Patent application, Ser. No. 62,868 filed Aug. 1 l, 1970.

An electric ammeter 200 is employed in the output of the bridge circuit14 for indicating changes in the magnitude of the differential pressurebeing applied across the pressure sensor 12 as shwon in FIG. 1 in termsof electrical units, for example, 4 to 20 milliamps which is equivalentto the O to 100 percent normal operating range that is selected for themeter 10.

If the previously described pressure of the high pressure fluid 44 andits related high pressure fill fluid 16 attempts to exceed a normaloperating pressure value the overload valve 28 will begin to move in adownward direction toward its closed or in a non-compressed engagementwith seating surface 110.

As the pressure on the sensor 12 increases due to an increase in thehigh pressure of the fluid acting on the top of the compliance bellows30 to compress it, the output of the sensor continues to increase untilthe overload valve 28 completes its previously mentioned fluidcompressing piston-like movement or until it reaches a position in whichthe convolutions at the top of the overload bellows are brought intomore compact nestled down relation with one another.

It should be noted that if a compliance bellows 30 is not used, and anabnormally high fluid pressure 44 and its panyitslt sb.fll qfi re. .16ha esent, then a meter measuring the output of the sensor woulderroneously indicate that the meter 10 was being operated within thenormal I00 percent scale range of the meter or operating at a levelwhich is below this range.

It should also be noted that the compliance bellows 30 will enable atrue reading to occur on the differential pressure measuring gage 200only when the differential pressure being sensed by the sensor 12 is infact within the normal 0 to I00 percent operating scale of the meter 10.g

If the compliance bellows 30 is not used then an overload pressurecondition on the low pressure side of the meter 10 occurs and theoperator is erroneously shown the value of the sensor output as being inthe normal operating range and at a range that is higher than the 100percent operating range value rather than showing him that the truevalue of the overload pressure is in fact below the operating range ofthe meter 10. H H g g g Since a compliance bellows 30 is provided whenthis overload pressure condition occurs, the output of the sensor 12will remain above the I00 percent non indicating level.

A suitable number of threaded connections which may be in the form offlat head machine screws, for example, 202, 204, 206, 208 are shownspaced about and passing through each of the support plates 146, 148 andbody to retain the-high and low fill fluids l6, 18 in sealed fluid tightrelationship between the barrier diaphragms 22, 24

Therefore, when the nut and bolt connections, for example, 158, 160;162, 164 and the heads 56, 58 are removed from the external surface ofthe support plate 146, 148 the barrier diaphragms 22, 24 which havetheir peripheral portions welded to their associated up o t latss 1165..v1.41? will arat unitary fluid tight unit. This unit in turn containsmodular parts e.g. 136 that can readily be removed and replaced withanother module. Such a meter construction affords a very rapid way inwhich the meter 10 can be repaired and therefore will reduce thedowntime loss in production that is necessitated when such a repair isrequired.

The overload compliance bellows assembly 30' and its associated partssuch as its stop member 130' shown in FIG. 3 is similar to and functionsin substantially the same manner as that already described for the samenamed parts under the description of FIG. 1.

The main difference is that the base 128 of the cylindrical supportblock 132' is mounted on a cylindrical bored out base portion2l0 of themeter body 20 and is retained in this position by a suitable number ofspaced apart threaded screw connections and holding down straps such asthe hold down strap 212 and the threaded screw connection 214.

Another difference which the apparatus shown in FIG. 3 discloses overthat shown in FIG. 1 is that the spring 216 is of a coil springconfiguration in FIG. 3 rather than being of the U-shaped configuration139 as shwon in FIG. 1.

Still another difference is that the semi-conductor wafer 218 forms anupper circular wall of chamber 220 to which the upper end of the coilspring 216 is fixedly connected and acts as a force loaded plate ratherthan being in the form of a cantilever beam as shown in FIG. 1.

A shield 222 fixedly mounted on the top surface of the cylindricalsupport block by a suitable epoxy or other cementing material isemployed which performs the same function as that already described forthe shield 141 of FIG. 1.

The other remaining parts having primes after the reference numeralsshown in FIG. 3 are merely to identify the parts already described inFIG. 1 which perform the same function as those parts in FIG. 1.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A transducer, comprising a meter body having cavities in spaced apartwalls thereof, a pair of opposied diaphragms each connected to adifferent one of said spaced apart walls and extending in spaced apartrelationship over the cavities contained therein to form a fluid tightcover for the spaced apart walls of the meter body, a first flexiblemember within a first one of said cavities, forming a first chamber, asecond flexible member forming a second chamber within a second one ofsaid cavities, said first and second chambers being adjacent to andspaced from a first one of said diaphragms, a sensing transducerpositioned within said second chamber and connected by means of springmeans to a movable portion of said second flexible member to convertchanges in displacement of said second flexible member into a force thatis applied to said sensing transducer, a first fluid under pressurewithin the space between said first one of said diaphragms and itsassociated meter wall to a first outer wall of said first flexiblemember, to the outer wall of said second flexible member and to an areasurrounding said sensing transducer, a passageway in said meter body toapply a second fluid under pressure within the space between said seconddiaphragm and its associated meter wall to the inside wall of saidflexible members, and an overload valve passing through a passagewaycontaining said second fluid and connected for movement with said firstflexible member and wherein said chamber formed by said second flexiblemember is operable to relieve the pressure of either one of said fluidsacting on said first flexible member whenever the pressure of either ofsaid fluids approaches an overload pressure condition.

2. The differential pressure responsive apparatus comprising a meterbody, a semi-conductor wafer positioned within said meter body having aresistive sensing element thereon to convert changes occurring in arelatively high and low incompressible fluid pressure under measurementinto an electric output signal of proprotional magnitude, an overloadvalve having a soft and hard seat and positioned in a first chamberformed by a body portion of the meter, said overload valve being mountedfor movement with a first flexible member forming a soft seat positionto a hard seated sealed off position with the meter body to cut off lowpressure fluid on a low pressure side of said meter from being appliedto said first flexible member during a condition in which said highincompressible fluid pressure approaches an overload pressure level, aflexible compliance member mounted within an aperatured wall formed inanother body portion of the meter, a spring means connecting theflexible compliance member to said wafer to convert changes indisplacement of the compliance member resulting from a change in thedifferential pressure of the fluids under measurement acting thereoninto a proportional force that is applied to said wafer, a secondchamber formed by the internal wall of said compliance member,passageways in said body for simultaneously applying said lowincompressible fluid pressure being applied to the internal wall of saidfirst flexible member and to the internal wall of the second chamber toexpand said flexible compliance member and thereby maintain the lowpressure fluid at a level that will not impair the sensing andtransmitting characteristics of the semi-conductor wafer and itsassociated resistive sensing element.

3. The apparatus as defined in claim 1 wherein the said sensingtransducer is a wafer formed of 'a single crystal silicon material andsaid compliance member has a stop means associated therewith to limitits displacement during said overload pressure condition and to therebylimit the magnitude of force that the spring means transmits to thewafer to a level that is below the rupture level of the wafer.

4. The apparatus as defined in claim 2 wherein the resistive sensor iscomprised of a fully active wheatstone bridge pattern and an adjustablestop means is mounted on said meter body to adjust the amount ofmovement that said flexible compliance member can be expanded orcompressed by said high and low incompressible fluid pressures.

5. The apparatus as defined in claim 2 wherein a substantiallymotionless electrical connection extends between the resistive sensingelement, through an electrical feed through member in the wall of themeter and through an electrical power transmitting circuit positionedoutside the meter and wherein said compliance member has a stop meansassociated therewith to limit its displacement during said overloadpressure condition and thereby limit the magnitude of force that thespring means transmits to the wafer to a level that is below the rupturelevel of the wafer.

6. The apparatus as defined in claim 2 wherein the overload valve has asecond soft and hard seat for engagement with a portion of said meterbody in a manner similar to but in a direction opposite to that in whichsaid soft and hard seat of said overload valve is closed when themagnitude of the low pressure fluid being applied to the inner wall ofsaid first flexible member exceeds an overload pressure condition andcauses said first flexible member to expand, the expansion of said firstflexible member being accompanied by a corresponding compression of saidflexible compliance member as said low over load incompressible fluidpressure is applied thereto and an adjustable stop associated with astationary base wall of said flexible compliance member is employed tolimit the displacement of the flexible compliance member during the timewhen either one of said overload fluid pressure conditions are present.

7. The apparatus as defined in claim 2 wherein a portion of the waferand its associated diffused resistive I sensing element is supported byone end of a hollow block, a wall forming one of said cavities in saidbody surrounds and is in spaced apart relation with said block, a sealis mounted in an outer wall of the block and extends outwardly intocontact with the wall forming said last mentioned cavity in the body,the open end of said flexible compliance member being supported by awall portion that forms an aperature in the other end of the hollowblock and said seal forming a fluid tight joint for separating said highincompressible fluid pressure acting on the sensor and the externalsurface of the compliance member from the low incompressible fluidpressure acting on the internal wall of the said compliance member.

8. The apparatus as defined in claim 1 wherein the said sensingtransducer is a wafer formed of a single crystal silicon material andsaid second flexible member has a stop means associated therewith tolimit its displacement during said overload pressure condition and tothereby limit the magnitude of force that the spring means transmits tothe wafer to a level that is below the rupture level of the wafer, saidsecond flexible member is a bellows, the spring means is of a coiledspring configuration having a first end fixedly connected to said waferand its other end fixedly connected to a plate member forming a closedend of said second flexible bellows, a first stop member is connectedfor movement with said plate member, a second stop member is adjustablymounted in a fixed position on a plate on which the open end of saidsecond flexible bellows is fixedly mounted, and said first stop memberbeing operably positioned for movement toward and into non-movableengagement with said second stop member when the second flexible memberis moved in one direction and said plate member forming the closed endof the bellows being operably positioned for movement toward and intonon-movable engagement with saidsecond stop member when the secondflexible member is moved in an opposite direction.

9. The apparatus as defined in claim 2 wherein the wafer is of a singlecrystal silicon material in which said resistive sensing element is adoped piezoresistive pattern and said compliance member has a stop meansassociated therewith to limit its displacement during said overloadpressure condition and thereby limit the magnitude of force that thebiasing means transmits to the wafer to a level that is below therupture level of the wafer, and an insulated supporting means to supportsaid compliance bellows, spring means and sensing element inelectrically isolated contact with said meter body to thereby preventleakage current passing from said wafer through said meter body toground.

10. The apparatus as defined in claim 2 wherein the wafer is of a singlecrystal silicon material in which said resistive sensing element is adoped piezoresistive pattern and said compliance member has a stop meansassociated therewith to limit its displacement during said overloadpressure condition and thereby limit the magnitude of force that thespring means transmits to the wafer to a level that is below the rupturelevel of the wafer, an insulated supporting means to support saidcompliance bellows, spring means and sensing element in electricallyisolated contact with said meter body to thereby prevent leakage currentpassing from said wafer through said meter body to ground, and whereinthe biasing means is of a U-shaped configuration.

1. A transducer, comprising a meter body having cavities in spaced apartwalls thereof, a pair of opposied diaphragms each connected to adifferent one of said spaced apart walls and extending in spaced apartrelationship over the cavities contained therein to form a fluid tightcover for the spaced apart walls of the meter body, a first flexiblemember within a first one of said cavities, forming a first chamber, asecond flexible member forming a second chamber within a second one ofsaid cavities, said first and second chambers being adjacent to andspaced from a first one of said diaphragms, a sensing transducerpositioned within said second chamber and connected by means of springmeans to a movable portion of said second flexible member to convertchanges in displacement of said second flexible member into a force thatis applied to said sensing transducer, a first fluid under pressurewithin the space between said first one of said diaphragms and itsassociated meter wall to a first outer wall of said first flexiblemember, to the outer wall of said second flexible member and to an areasurrounding said sensing transducer, a passageway in said meter body toapply a second fluid under pressure within the space between said seconddiaphragm and its associated meter wall to the inside wall of saidflexible members, and an overload valve passing through a passagewaycontaining said second fluid and connected for movement with said firstflexible member and wherein said chamber formed by said second flexiblemember is operable to relieve the pressure of either one of said fluidsacting on said first flexible member whenever the pressure of either ofsaid fluids approaches an overload pressure condition.
 2. Thedifferential pressure responsive apparatus comprising a meter body, asemi-conductor wafer positioned within said meter body having aresistive sensing element thereon to convert changes occurring in arelatively high and low incompressible fluid pressure under measurementinto an electric output signal of proprotional magnitude, an overloadvalve having a soft and hard seat and positIoned in a first chamberformed by a body portion of the meter, said overload valve being mountedfor movement with a first flexible member forming a soft seat positionto a hard seated sealed off position with the meter body to cut off lowpressure fluid on a low pressure side of said meter from being appliedto said first flexible member during a condition in which said highincompressible fluid pressure approaches an overload pressure level, aflexible compliance member mounted within an aperatured wall formed inanother body portion of the meter, a spring means connecting theflexible compliance member to said wafer to convert changes indisplacement of the compliance member resulting from a change in thedifferential pressure of the fluids under measurement acting thereoninto a proportional force that is applied to said wafer, a secondchamber formed by the internal wall of said compliance member,passageways in said body for simultaneously applying said lowincompressible fluid pressure being applied to the internal wall of saidfirst flexible member and to the internal wall of the second chamber toexpand said flexible compliance member and thereby maintain the lowpressure fluid at a level that will not impair the sensing andtransmitting characteristics of the semi-conductor wafer and itsassociated resistive sensing element.
 3. The apparatus as defined inclaim 1 wherein the said sensing transducer is a wafer formed of asingle crystal silicon material and said compliance member has a stopmeans associated therewith to limit its displacement during saidoverload pressure condition and to thereby limit the magnitude of forcethat the spring means transmits to the wafer to a level that is belowthe rupture level of the wafer.
 4. The apparatus as defined in claim 2wherein the resistive sensor is comprised of a fully active wheatstonebridge pattern and an adjustable stop means is mounted on said meterbody to adjust the amount of movement that said flexible compliancemember can be expanded or compressed by said high and low incompressiblefluid pressures.
 5. The apparatus as defined in claim 2 wherein asubstantially motionless electrical connection extends between theresistive sensing element, through an electrical feed through member inthe wall of the meter and through an electrical power transmittingcircuit positioned outside the meter and wherein said compliance memberhas a stop means associated therewith to limit its displacement duringsaid overload pressure condition and thereby limit the magnitude offorce that the spring means transmits to the wafer to a level that isbelow the rupture level of the wafer.
 6. The apparatus as defined inclaim 2 wherein the overload valve has a second soft and hard seat forengagement with a portion of said meter body in a manner similar to butin a direction opposite to that in which said soft and hard seat of saidoverload valve is closed when the magnitude of the low pressure fluidbeing applied to the inner wall of said first flexible member exceeds anoverload pressure condition and causes said first flexible member toexpand, the expansion of said first flexible member being accompanied bya corresponding compression of said flexible compliance member as saidlow over load incompressible fluid pressure is applied thereto and anadjustable stop associated with a stationary base wall of said flexiblecompliance member is employed to limit the displacement of the flexiblecompliance member during the time when either one of said overload fluidpressure conditions are present.
 7. The apparatus as defined in claim 2wherein a portion of the wafer and its associated diffused resistivesensing element is supported by one end of a hollow block, a wallforming one of said cavities in said body surrounds and is in spacedapart relation with said block, a seal is mounted in an outer wall ofthe block and extends outwardly into contact with the wall forming saidlast mentioned cavity in the body, the open end of said fLexiblecompliance member being supported by a wall portion that forms anaperature in the other end of the hollow block and said seal forming afluid tight joint for separating said high incompressible fluid pressureacting on the sensor and the external surface of the compliance memberfrom the low incompressible fluid pressure acting on the internal wallof the said compliance member.
 8. The apparatus as defined in claim 1wherein the said sensing transducer is a wafer formed of a singlecrystal silicon material and said second flexible member has a stopmeans associated therewith to limit its displacement during saidoverload pressure condition and to thereby limit the magnitude of forcethat the spring means transmits to the wafer to a level that is belowthe rupture level of the wafer, said second flexible member is abellows, the spring means is of a coiled spring configuration having afirst end fixedly connected to said wafer and its other end fixedlyconnected to a plate member forming a closed end of said second flexiblebellows, a first stop member is connected for movement with said platemember, a second stop member is adjustably mounted in a fixed positionon a plate on which the open end of said second flexible bellows isfixedly mounted, and said first stop member being operably positionedfor movement toward and into non-movable engagement with said secondstop member when the second flexible member is moved in one directionand said plate member forming the closed end of the bellows beingoperably positioned for movement toward and into non-movable engagementwith said second stop member when the second flexible member is moved inan opposite direction.
 9. The apparatus as defined in claim 2 whereinthe wafer is of a single crystal silicon material in which saidresistive sensing element is a doped piezoresistive pattern and saidcompliance member has a stop means associated therewith to limit itsdisplacement during said overload pressure condition and thereby limitthe magnitude of force that the biasing means transmits to the wafer toa level that is below the rupture level of the wafer, and an insulatedsupporting means to support said compliance bellows, spring means andsensing element in electrically isolated contact with said meter body tothereby prevent leakage current passing from said wafer through saidmeter body to ground.
 10. The apparatus as defined in claim 2 whereinthe wafer is of a single crystal silicon material in which saidresistive sensing element is a doped piezoresistive pattern and saidcompliance member has a stop means associated therewith to limit itsdisplacement during said overload pressure condition and thereby limitthe magnitude of force that the spring means transmits to the wafer to alevel that is below the rupture level of the wafer, an insulatedsupporting means to support said compliance bellows, spring means andsensing element in electrically isolated contact with said meter body tothereby prevent leakage current passing from said wafer through saidmeter body to ground, and wherein the biasing means is of a U-shapedconfiguration.